1. Field of the Invention
The present invention relates to a surface acoustic wave device comprising an interdigital electrode on a single crystal substrate.
2. Discussion of the Background
In recent years, mobile communication terminal equipment inclusive of cellular telephones has been rapidly popularized. This terminal equipment is particularly desired to be reduced in size and weight for reason of portability. To achieve size and weight reductions for the terminal equipment, electronic parts used therewith, too, should be essentially reduced in size and weight. For this reason, surface acoustic wave devices favorable for size and weight reductions, i.e., surface acoustic wave filters are often used for high- and intermediate-frequency parts of the terminal equipment. A surface acoustic wave device has on the surface of a piezoelectric substrate an interdigital electrode for exciting, receiving, reflecting, and propagating surface acoustic waves.
Among characteristics important to a piezoelectric substrate used for surface acoustic wave devices, there are the surface wave velocity of surface acoustic waves (SAW velocity), the temperature coefficient of a center frequency in the case of filters and of a resonance frequency in the case of resonators (the temperature coefficient of frequency: TCF), and an electromechanical coupling factor (k.sup.2). Set out in Table 1 are the characteristics of various piezoelectric substrate known so far for surface acoustic wave devices. Hereinafter, these piezoelectric substrates will be referred to by the symbols used in Table 1. In this regard, it is to be noted that TCV (the temperature coefficient of SAW velocity) is a quantity representing the temperature dependence of the velocity of surface acoustic waves (the SAW velocity); that is, it has a value equivalent to that of the aforesaid TCF representing the temperature dependence of the center or resonance frequency. A large TCV value implies that the center frequency of a surface acoustic wave filter varies significantly with temperature.
TABLE 1 __________________________________________________________________________ Propagation SAW Velocity k.sup.2 TCV Symbol Composition Cut Angle Direction (m/s) (ppm/.degree. C.) __________________________________________________________________________ 128LN LiNbO.sub.3 128.degree.-Rotation Y X 3992 5.5 -74 64LN 64.degree.-Rotation Y X -79.3 LT112 3288degree.-Rotation Y 0.64 -18 36LT 36.degree.-Rotation Y X -45.7 ST Crystal Quartz 0 (primary coef.) BGO (100) GeO.sub.20 (011) -1222 __________________________________________________________________________
As can be seen from Table 1, 64LN and 36LT have an SAW velocity of 4,000 m/s or higher, and so are suitable to construct filters for high-frequency parts of terminal equipment. Referring now to the reason for this, various systems are practically employed for mobile communications represented by cellular telephones all over the world, and are all used at frequencies of the order of 1 GHz. Accordingly, filters used for high-frequency parts of terminal equipment have a center frequency of approximately 1 GHz. Surface acoustic wave filters have a center frequency substantially proportional to the SAW velocities of piezoelectric substrates used but almost inversely proportional to the widths of electrode fingers formed on Ad substrates. To enable such filters to be operated at high frequencies, therefore, it is preferable to resort to substrates having high SAW velocities, for instance, 64LN, and 36LT. Also, wide passband widths of 20 MHz or more are required for filters used on high-frequency parts. To achieve such broad passbands, however, it is essentially required that piezoelectric substrates have a large electromechanical coupling factor k.sup.2. For these reasons, much use is made of 64LN, and 36LT.
On the other hand, a frequency band of 70 to 300 MHz is used as an intermediate frequency for mobile terminal equipment. When a filter using this frequency band as a center frequency is constructed with the use of a surface acoustic wave device, the use of the aforesaid 64LN or 36LT as a piezoelectric substrate causes the width of an electrode finger formed on the substrate to be much larger than that of the aforesaid filter used for a high-frequency part.
This will now be explained with reference to roughly calculated specific values. Here let d represent the width of an electrode finger of a surface acoustic wave transducer that forms a surface acoustic wave filter, f.sub.0 indicate the center frequency of the surface acoustic wave filter, and V denote the SAW velocity of the piezoelectric substrate used. For these values, equation (1) then holds roughly EQU f.sub.0 =V/(4d) (1)
If a surface acoustic wave filter having a center frequency of 1 GHz is constructed on the assumption that the SAW velocity is 4,000 m/s, then the width of its electrode finger is calculated from equation (1) to be EQU d=4,000 (m/s)/[(4.times.1,000 (MHz)]=1 .mu.m
on the other hand, when an intermediate-frequency filter having a center frequency of 100 MHz is constructed using this piezoelectric substrate having an SAW velocity of 4,000 m/s, the electrode finger width required for this is given by EQU d=4,000 (m/s)/[(4.times.100 (MHz)]=10 .mu.m
Thus, the required electrode finger width is 10 times as large as that for the high-frequency part filter. A large electrode finger width implies that a surface acoustic wave device itself becomes large. To make a surface acoustic wave intermediate-frequency filter small, therefore, it is necessary to use a piezoelectric substrate having a low SAW velocity V, as can be appreciated from the aforesaid equation (1).
Among piezoelectric substrates known to have very limited SAW velocity, there is BGO such as one already referred to in the aforesaid Table 1. A BGO piezoelectric substrate has an SAW velocity of 1,681 m/s. However, the BGO piezoelectric substrate is unsuitable to construct an intermediate- frequency filter for extracting one channel signal alone, because its temperature coefficient of SAW velocity or its TCV is as large as -122 ppm/.degree. C. The reason is that TCV is the quantity indicative of the temperature dependence of SAW velocity as already noted, and that a large TCV value implies that the center frequency of the surface acoustic wave filter varies largely with temperature, as can again be seen from the aforesaid equation (1). Thus, large TCV is unsuitable for an intermediate-frequency filter because undesired signals may possibly be extracted from other channel adjacent to the desired channel.
Among piezoelectric substrates known to have relatively low SAW velocity there is ST quartz crystal such as one referred to in the aforesaid Table 1. The ST quartz crystal is suitable to construct an intermediate-frequency filter because its temperature coefficient of SAW velocity or its TCV is almost zero (with a primary temperature coefficient a of zero). For this reason, most of intermediate-frequency surface acoustic wave filters used so far for mobile communication terminal equipment are constructed of ST quartz crystal piezoelectric substrates.
However, the SAW velocity of the ST quartz crystal substrate is 3,158 m/s or is not on a sufficiently reduced level, and so presents some limitation on size reductions.
In addition, the electromechanical coupling factor k.sup.2 of the ST quartz crystal is 0.14%, and so is relatively small. Small k.sup.2 implies that only a filter having a narrow passband is achievable. Adopted mainly so far for mobile communications, that is, cellular telephones are analog systems with a very narrow channel band width of, for instance, 12.5 kHz according to the Japanese NTT standard, 30 kHz according to the U.S. AMPS standard, and 25 kHz according to the European TACS standard. Thus, the fact that the aforesaid ST quartz crystal has a small electromechanical coupling factor k.sup.2 has offered no problem whatsoever. In recent years, however, digital mobile communication systems have been developed, put to practical use, and so rapidly widespread in view of making effective use of frequency resources, compatibility with digital data communications, etc. The channel width of this digital system is very wide, for instance, 200 kHz and 1.7 MHz in the European cellular telephone GSM and cordless telephone DECT modes, respectively. If ST quartz crystal substrates are used for surface acoustic wave filters, it is then difficult to construct such wide-band intermediate-frequency filters using them.
On the other hand, it is known that the electromechanical coupling factor of a surface acoustic surface device can be increased by forming a piezoelectric film made up of zinc oxide, tantalum oxide, CdS or the like on the surface of a piezoelectric substrate made up of LiNbO.sub.3 or the like, as typically set forth in JP-A 8-204499. However, a conventional piezoelectric substrate such as an LiNbO.sub.3 substrate is not preferable because its temperature coefficient of SAW velocity, TCV, is negative, and so its overall TCV is greatly shifted to a negative side when a zinc oxide film is provided thereon.
As explained above, a problem with a conventional surface acoustic wave device is that when a piezoelectric substrate such as the aforesaid 64LN, 36LT or the like is used, it is possible to make its passband broad, but device size becomes large because that substrate has high SAW velocity. Another problem is that when the aforesaid BGO substrate having low SAW velocity is used to achieve device size reductions, good-enough selectivity is not obtained due to too large a temperature coefficient of SAW velocity or TCV. In either case, characteristics good enough for any intermediate- frequency surface acoustic wave filter are unachievable.
The ST quartz crystal substrate having a small temperature coefficient of SAW velocity, TCV, presents some limitation on size reductions due to the fact that its SAW velocity is not sufficiently reduced, and makes it difficult to achieve wide band due to the fact that its electro- mechanical coupling factor is relatively small.